Pushbutton locks are well known and find many uses. One such lock is employed as a lid lock on the Keysafe brand lockbox, marketed by the present assignee and described in U.S. Pat. No. 4,936,894 and in U.S. Pat. No. 5,768,921 (incorporated herein by reference).
The patented lock includes plural elongated buttons, each of which has, on the sides thereof, plural cut-outs, or "gates." A case contains the lock and defines plural bores within which the buttons travel. The buttons are urged out a front cover of the case (i.e. towards the user) by biasing springs. Plural latching members cooperate with features on the buttons to hold same in a depressed position against the force of the biasing springs when the buttons are pushed in by an operator of the lock.
The lock further includes a locking bolt which is coupled to a checker plate. The checker plate defines plural edges that engage the sides of the buttons and are received in the gates therein if the buttons are properly positioned. The checker plate is slidably mounted in a track within the case and is normally urged to a first end of the track by another biasing spring. When the checker is at this first end of its travel, the bolt is in its locked position.
A knob is coupled to the checker plate and can be operated to urge the checker plate towards a second end of its track. However, the checker plate is prevented from moving to the second end of its track if the portions of the buttons engaged by the edges of the checker plate do not include gates.
By the foregoing arrangement, all of the buttons must be properly positioned (in vs. out, and rotationally correct) if the checker plate is to be moved to its second position so as to move the bolt to the unlocking position.
In accordance with a preferred embodiment of the present invention, various enhancements are provided to a pushbutton lock to enhance its security. One enhancement is the provision of button tips which are mounted on the ends of the button assemblies and are spring-loaded to protrude from the face of the lock, regardless of whether the underlying button is "in" or "out." By this arrangement, a bystander cannot readily see the button combination by the pattern of depressed buttons. This arrangement also reduces tactile feedback between the underlying button assembly and the externally available button tips, increasing resistance to pick attacks.
A further enhancement is the provision of button bars between adjacent pairs of button tips. Whenever any of the button tips is manually depressed, the corresponding button bar is pressed into engagement with a corresponding notch in a detent member (called the "centipede" in the issued patent). Such keying of the detent member prevents the lock bolt (more particularly the checker plate connected thereto) from movement. This arrangement further increases lock security by preventing an attacker from experimentally pressing buttons and judging the tactile effect thereon of attempted operations of the unlocking knob.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.